1. Field of the Invention
The present invention relates to a surface acoustic wave device, a method for making such a surface acoustic wave device, and a communication apparatus including such a surface acoustic wave device. In particular, the present invention relates to a surface acoustic wave device prepared by a flip chip process, in which metal bumps are disposed on electrode pads provided on a piezoelectric substrate of a surface acoustic wave element, and the electrode pads are connected to electrode pattern sections disposed in the interior of an electronic component package made of ceramic or other suitable material via the metal bumps.
2. Description of the Related Art
With recent trends towards compactness and versatility of communication devices such as portable communication terminals, further reductions in size and particularly in height are required for electronic components mounted therein. One of the electronic components used in such communication devices is a surface acoustic wave device.
The surface acoustic wave device used includes a surface acoustic wave element and an electronic component package to accommodate the surface acoustic wave element. The surface acoustic wave element includes a piezoelectric substrate. Interdigital electrodes and electrode pads for inputting and outputting electrical signals to and from the interdigital electrodes are provided on the piezoelectric substrate. The electronic component package is defined by a box made of a ceramic material such as alumina.
Flip chip processes are used for mounting surface acoustic wave elements in electronic component packages to reduce the size and particularly height thereof. In the flip chip process, electrode pattern sections are provided on an inner surface of the electronic component package at positions corresponding to the electrode pads of the surface acoustic wave device.
The flip chip process includes the following steps:
(1) Metal bumps are formed on respective electrode pads of the surface acoustic wave element;
(2) The metal bumps are sandwiched between the electrode pads and respective electrode pattern sections on the mounting surface of the electronic component package such that the functional surface of the surface acoustic wave element faces the electronic component package;
(3) The surface acoustic wave element is pressed toward the electronic component package to apply a load to the metal bumps; and
(4) The metal bump connections of the metal bumps are connected to the respective electrode pattern sections and the respective electrode pads on the mounting surface of the electronic component package by the load.
Procedures often used to mount the surface acoustic wave element in the electronic component package by the flip chip process include simultaneous application of a load and ultrasonic waves and simultaneous application of a load, ultrasonic waves, and heat.
In the surface acoustic wave device made with the flip chip process, the connections between the metal bumps on the surface acoustic wave element and the respective electrode pattern sections on the electronic component package must have high electrical conductivity and high mechanical strength to fix the surface acoustic wave element on the electronic component package. In other words, the connections must be highly reliable.
In known flip chip processes, however, cracks often form on the piezoelectric substrate at locations the electrode pads. These cracks decrease the bonding strength between the surface acoustic wave element and the electronic component package. As a result, the surface acoustic wave device cannot withstand dropping shock, impact, and thermal stress.
The method for making a surface acoustic wave filter and the characteristics and reliability thereof with a flip chip process are reported in TECHNICAL REPORT OF IEICE. US95-26, EMD95-22, CPM95-34 (1995-07), p. 47, titled xe2x80x9cFlip Chip GHz-Band SAW (Surface Acoustic Wave) Filterxe2x80x9d; Hiromi YATSUTA, Masatoshi OGURI, and Taira HORISHIMA (Japan Radio Co., Ltd.).
In this paper, a load, heat, and ultrasonic waves are applied to connect metal bumps on a chip (piezoelectric substrate) of the surface acoustic wave with the respective electrode pattern sections of the electronic component package. Furthermore, the report describes that an excess load or excess ultrasonic wave power in the flip chip process causes cracking on the chip at locations of the electrode pads.
In general, it is believed that an increased load, increased ultrasonic power, or an increased temperature is effective in improving the bonding strength of the metal bump of the surface acoustic wave device. However, an excess load or excess ultrasonic wave power often causes surface cracking of the piezoelectric substrate, as described above.
Such surface cracking of the piezoelectric substrate decreases the bonding strength between the surface acoustic wave element and the electronic component package. Hence, the surface acoustic wave device cannot withstand dropping shock, impact, and thermal stress.
As a result, a communication apparatus using such a surface acoustic wave device is also unreliable.
In order to overcome the above-described problems, preferred embodiments of the present invention provide a highly reliable surface acoustic wave made by a flip chip process.
Another preferred embodiment of the present invention provides a method for making the surface acoustic wave by a flip chip process.
Another preferred embodiment of the present invention provides a communication apparatus including the surface acoustic wave device.
According to a first preferred embodiment of the present invention, a surface acoustic wave device includes a surface acoustic wave element including a piezoelectric substrate which has interdigital electrodes and electrode pads thereon, the electrode pads input and output electrical signals to and from the respective interdigital electrodes, an electronic component package to support the surface acoustic wave element, the electronic component package including electrode pattern sections for inputting and outputting electrical signals, and metal bump connections to electrically connect the electrode pads to the respective electrode pattern sections, wherein the electrode pads include aluminum as the major component and copper as a minor component, the copper content being at least about 3.5 percent by weight. Herein, the interdigital electrodes are also referred to as interdigital transducers (IDTs).
Electrical signals input to the piezoelectric substrate through the IDTs are transformed into surface acoustic waves, which are retransformed into electrical signals by the IDTs. Thus, this surface acoustic wave device functions as a filter that transmits only electrical signals within a desired pass band.
Since the metal bump connections for electrically connecting the electrode pads to the electrode pattern sections are provided, the electrode pads and the electrode pattern sections are disposed within the surface area of the surface acoustic wave element, hence, the volume of the package is greatly reduced as compared with packaging by known wire bonding methods.
Since the surface acoustic wave element is supported in the electronic component package via the metal bump connections, the surface of the surface acoustic wave element having the IDTs mounted thereon is spaced from the electronic component package by forming the electrode pads on the same surface, resulting in stable operation of the IDTs.
As described above, the electrode pads include aluminum as the major component and copper as a minor component and the copper content is at least about 3.5 percent by weight. Such an electrode pad composition reduces cracking on the piezoelectric substrate at the locations of the electrode pads.
Furthermore, the above-mentioned configuration minimizes breakage of the surface acoustic wave element and prevents a decrease in bonding strength between the surface acoustic wave device and the electronic component package, resulting in greatly improved production yield, greatly reduced defect rate, and greatly improved product reliability.
Preferably, the metal bump connections include elemental gold or an alloy including gold as the primary component. Since gold and gold alloy have high ductility, the gold bump connections provide secure electrical and mechanical connections between the electrode pads and the respective electrode pattern sections. Thus, the surface acoustic wave element is more securely bonded to the electronic component package which prevents the surface acoustic wave element from breaking.
According to another preferred embodiment of the present invention, a method for making the above-described surface acoustic wave device includes the steps of sandwiching a metal bump between each electrode pad and the corresponding electrode pattern section on the electronic component package, and forming the corresponding metal bump connection from the metal bump by applying ultrasonic waves.
In this method, the surface acoustic wave element is securely bonded to the electronic component package in the surface acoustic wave device which prevents the surface acoustic wave element from breaking. Furthermore, the metal bump connections are formed with the metal bumps by applying ultrasonic waves. Since this ultrasonic process reduces thermal damage of other components such as the IDTs as compared with heating processes such as soldering, the resulting surface acoustic wave device is highly reliable.
In the above-described method, the metal bump is placed on the corresponding electrode pad before the metal bump is sandwiched between the electrode pad and the electrode pattern section. Since the electrode pad is exposed at the surface of the piezoelectric substrate, the metal bump is more easily placed on the corresponding electrode pad as compared with providing a metal bump on an electrode pattern section in an electronic component package. Thus, according to this additional step, the production process of the surface acoustic wave device is simplified with high reliability.
According to another preferred embodiment of the present invention, a communication apparatus includes the above-described surface acoustic wave device. As a result, the communication apparatus is also highly reliable.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.